1. Field of the Invention
The present invention relates to a semiconductor memory device, for example, a floating-body-cell (FBC) memory device that stores information by accumulating majority carriers in a floating body of a field-effect transistor (FET).
2. Related Art
Recently, an FBC memory device is expected as a semiconductor memory device that replaces a dynamic random-access memory (DRAM). The FBC memory device is configured as follows. An FBC memory cell constituted by an FET including a floating body (hereinafter, also “body region”) is formed on a silicon-on insulator (SOI) substrate. The FBC memory cell stores therein data “1” or “0” according to the number of carriers accumulated in the body region.
The FBC memory constituted by the FET has a problem of charge pumping phenomenon (see Non-Patent Document 1). If the FBC memory cell is an n-type FET, parts of electrons in an inversion layer are trapped by an interface state on an interface between a gate oxide film and the body region when the FBC memory cell is turned on. The holes accumulated in the body region are recombined with the electrons and disappear. If a word line is activated between a refresh operation and a next refresh operation, data is not written back to an unselected FBC memory cell. Due to this, if the unselected memory cell is repeatedly turned on and off during reading or writing of the data from or to a selected memory cell, the number of holes accumulated in the body region of the unselected memory cell that stores the data “1” therein gradually decreases. As a result, the data “1” in the unselected memory cell is changed to data “0”.
Accordingly, the FBC memory device is not a destructive read-out cell differently from the DRAM, but is not a complete non-destructive read-out cell. In other words, it has been made clear that the FBC memory device is a quasi non-destructive read-out cell.
The charge pumping phenomenon influences not only ordinary memory cells but also dummy cells. The influence of the charge pumping phenomenon on the dummy cell is rather greater than that on the ordinary memory cell because the number of times of reading data from the dummy cell is larger than that of reading data from the ordinary memory cell.
Conventionally, the charge pumping phenomenon influencing the dummy cell is dealt with by providing an FET to which the data “1” is written for every other bit line. However, if the FET is provided for every other bit line, a circuit scale is disadvantageously made large and a chip size of the memory device is disadvantageously increased.